'\"macro stdmacro
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.TH PMAPI 3 "PCP" "Performance Co-Pilot"
.SH NAME
\f3PMAPI\f1 \- introduction to the Performance Metrics Application Programming Interface
.SH "C SYNOPSIS"
.ft 3
#include <pcp/pmapi.h>
.sp
.ft 1
\& ... assorted routines ...
.ft 3
.sp
cc ... \-lpcp
.ft 1
.SH DESCRIPTION
.de CW
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.\}
Within the framework of the Performance Co-Pilot (PCP), client
applications are developed using the
Performance Metrics Application Programming Interface (PMAPI) that
defines a procedural interface with services suited to the development
of applications with a particular interest in performance metrics.
.PP
This description presents an overview of the PMAPI and the
context in which PMAPI applications are run.
The PMAPI is more fully described in the
.IR "Performance Co-Pilot Programmer's Guide" ,
and the manual pages for the individual PMAPI routines.
.SH "PERFORMANCE METRICS \- NAMES AND IDENTIFIERS"
For a description of the Performance Metrics Name Space (PMNS)
and associated terms and concepts,
see
.BR PCPIntro (1).
.PP
Not all PMIDs need be represented in the PMNS of
every application.
For example, an application which monitors disk
traffic will likely use a name space which references only the PMIDs
for I/O statistics.
.PP
Applications which use the PMAPI may have independent
versions of a PMNS, constructed from an initialization file when the
application starts; see
.BR pmLoadASCIINameSpace (3),
.BR pmLoadNameSpace (3),
and
.BR PMNS (5).
.PP
Internally (below the PMAPI) the implementation of the
Performance Metrics Collection System
(PMCS) uses only the PMIDs, and a PMNS
provides an external mapping from a hierarchic taxonomy of names to
PMIDs that is
convenient in the context of a particular system or particular use of
the PMAPI.
For the applications programmer,
the routines
.BR pmLookupName (3)
and
.BR pmNameID (3)
translate between names in a PMNS and PMIDs, and vice versa.
The PMNS may be traversed using
.BR pmGetChildren "(3) and" pmTraversePMNS .
The
.BR pmFetchGroup (3)
functions combine metric name lookup, fetch, and conversion operations.
.SH "PMAPI CONTEXT"
An application using the PMAPI may manipulate several concurrent contexts,
each associated with a source of performance metrics, e.g. \c
.BR pmcd (1)
on some host, or a set of archive logs of performance metrics as created by
.BR pmlogger (1).
.PP
Contexts are identified by a ``handle'', a small integer value that is returned
when the context is created; see
.BR pmNewContext (3)
and
.BR pmDupContext (3).
Some PMAPI functions require an explicit ``handle'' to identify
the correct context, but more commonly the PMAPI function is
executed in the ``current'' context.
The current context may be discovered using
.BR pmWhichContext (3)
and changed using
.BR pmUseContext (3).
.PP
If a PMAPI context has not been explicitly established
(or the previous current context has been closed using
.BR pmDestroyContext (3))
then the current PMAPI context is undefined.
.PP
In addition to the source of the performance metrics, the context
also includes the instance profile and collection time (both described below)
which controls
how much information is returned, and when the information was collected.
.SH "INSTANCE DOMAINS"
When performance metric values are returned across the PMAPI to a
requesting application, there may be more than one value for a
particular metric.
Multiple values, or
.BR instances ,
for a single metric
are typically the result of instrumentation being implemented for each
instance of a set of similar components or services in a system, e.g.
independent counts for each CPU, or each process, or each disk, or each
system call type, etc.
This multiplicity of values is not enumerated in
the name space but rather, when performance metrics are delivered
across the PMAPI by
.BR pmFetch (3),
the format of the result accommodates values for one
or more instances, with an instance-value pair
encoding the metric value for a particular
instance.
.PP
The instances are identified by an internal identifier assigned
by the agent responsible for instantiating the values for the
associated performance metric.
Each instance identifier has a corresponding external instance identifier
name (an ASCII string).
The routines
.BR pmGetInDom (3),
.BR pmLookupInDom (3)
and
.BR pmNameInDom (3)
may be used to enumerate all instance identifiers, and to
translate between internal and external instance
identifiers.
.PP
All of the instance identifiers for a particular performance metric
are collectively known as an instance domain.
Multiple performance metrics may share the same instance domain.
.PP
If only one instance is ever available for a particular performance
metric, the instance identifier
in the result from
.BR pmFetch (3)
assumes the special value
.B PM_IN_NULL
and may be ignored by the
application, and only one instance-value pair appears in the result
for that metric.
Under these circumstances, the associated instance domain (as returned
via
.BR pmLookupDesc (3))
is set to
.B PM_INDOM_NULL
to indicate that values for this metric are singular.
.PP
The difficult issue of
transient performance metrics (e.g. per-filesystem information, hot-plug
replaceable hardware modules, etc.) means that repeated requests for
the same PMID may return different numbers of values, and/or some
changes in the particular instance identifiers returned.
This means
applications need to be aware that metric instantiation is guaranteed
to be valid at the time of collection only.
Similar rules apply to the
transient semantics of the associated metric values.
In general
however, it is expected that the bulk of the performance metrics will
have instantiation semantics that are fixed over the execution
life-time of any PMAPI client.
.SH "THE TYPE OF METRIC VALUES"
The PMAPI supports a wide range of format and type encodings for
the values of performance metrics, namely signed and unsigned integers,
floating point numbers, 32-bit and 64-bit encodings of all of the above,
ASCII strings (C-style, NULL byte terminated), and arbitrary aggregates of
binary data.
.PP
The
.CW type
field in the
.CW pmDesc
structure returned by
.BR pmLookupDesc (3)
identifies the format and type of the values for a particular
performance metric within a particular PMAPI context.
.PP
Note that the encoding of values for a particular performance metric
may be different for different PMAPI contexts, due to differences
in the underlying implementation for different contexts.
However it is expected that the vast majority of performance metrics
will have consistent value encoding across all versions of all
implementations, and hence across all PMAPI contexts.
.PP
The PMAPI supports routines to automate the handling
of the various value formats and types, particularly for
the common case where conversion to a canonical format is
desired, see
.BR pmExtractValue (3)
and
.BR pmPrintValue (3).
.SH "THE DIMENSIONALITY AND SCALE OF METRIC VALUES"
Independent of how the value is encoded, the
value for a performance metric is assumed to be drawn from a set of values that
can be described in terms of their dimensionality and scale by a compact
encoding as follows.
The dimensionality is defined by a power, or index, in
each of 3 orthogonal dimensions, namely Space, Time and Count
(or Events, which are dimensionless).
For example I/O throughput might be represented as
Space/Time, while the
running total of system calls is Count, memory allocation is Space and average
service time is Time/Count.
In each dimension there are a number
of common scale values that may be used to better encode ranges that might
otherwise exhaust the precision of a 32-bit value.
This information is encoded
in the
.CW pmUnits
structure which is embedded in the
.CW pmDesc
structure returned from
.BR pmLookupDesc (3).
.PP
The routine
.BR pmConvScale (3)
is provided to convert values in
conjunction with the
.CW pmUnits
structures that defines the dimensionality and scale of the values for a
particular performance metric as returned from
.BR pmFetch (3),
and the desired dimensionality and scale of
the value the PMAPI client wishes to manipulate.
Alternatively, the
.BR pmFetchGroup (3)
functions can perform data format and unit conversion operations,
specified by textual descriptions of desired unit / scales.
.SH "INSTANCE PROFILE"
The set of instances for performance metrics returned from a
.BR pmFetch (3)
call may be filtered or restricted using an instance profile.
There is one instance profile for each PMAPI context the application
creates,
and each instance profile may include instances from one or more
instance domains.
.PP
The routines
.BR pmAddProfile (3)
and
.BR pmDelProfile (3)
may be used to dynamically adjust the instance profile.
.SH "COLLECTION TIME"
For each set of values for performance metrics returned
via
.BR pmFetch (3)
there is an associated ``timestamp''
that serves to identify when the performance metric
values were collected; for metrics being delivered from
a real-time source (i.e. \c
.BR pmcd (1)
on some host) this would typically be not long before they
were exported across the PMAPI, and for metrics being delivered
from a set of archive logs, this would be the time when the metrics
were written into the archive log.
.PP
There is an issue here of exactly
when individual metrics may have been collected, especially given
their origin in potentially different Performance Metric Domains, and
variability in the metric updating frequency at the lowest level of the
Performance Metric Domain.
The PMCS opts for the pragmatic approach,
in which the PMAPI implementation undertakes to return all of the
metrics with values accurate as of the timestamp, to the best of our
ability.
The belief is that the inaccuracy this introduces is small,
and the additional burden of accurate individual timestamping for each
returned metric value is neither warranted nor practical (from an
implementation viewpoint).
.PP
Of course, in the case of collection of
metrics from multiple hosts the PMAPI client must assume the
sanity of the timestamps is constrained by the extent to which clock
synchronization protocols are implemented across the network.
.PP
A PMAPI application may call
.BR pmSetMode (3)
to vary the requested collection time, e.g. to rescan performance
metrics values from the recent past, or to ``fast-forward'' through
a set of archive logs.
.SH "GENERAL ISSUES OF PMAPI PROGRAMMING STYLE"
Across the PMAPI, all arguments and results involving a
``list of something'' are declared to be arrays with an associated argument or
function value to identify the number of elements in the list.
This has been done to avoid both the
.BR varargs (3)
approach and sentinel-terminated lists.
.PP
Where the size of a result is known at the time of a call, it
is the caller's responsibility to allocate (and possibly free) the
storage, and the called function will assume the result argument is of
an appropriate size.
Where a result is of variable size and that size
cannot be known in advance (e.g. for
.BR pmGetChildren (3),
.BR pmGetInDom (3),
.BR pmNameInDom (3),
.BR pmNameID (3),
.BR pmLookupLabels (3),
.BR pmLookupText (3)
and
.BR pmFetch (3))
the PMAPI implementation uses a range of dynamic
allocation schemes in the called routine, with the caller
responsible for subsequently releasing the storage when
no longer required.
In some cases this simply involves calls to
.BR free (3),
but in others (most notably for the result from
.BR pmFetch (3)),
special routines (e.g. \c
.BR pmFreeResult (3)
and
.BR pmFreeLabelSets (3))
should be used to release the storage.
.PP
As a general rule, if the called routine returns
an error status then no allocation will have been
done, and any pointer to a variable sized result is undefined.
.SH DIAGNOSTICS
Where error conditions may arise, the functions that comprise the PMAPI conform to a single, simple
error notification scheme, as follows;
.IP + 3n
the function returns an integer
.IP + 3n
values >= 0 indicate no error, and perhaps some positive status,
e.g. the number of things really processed
.IP + 3n
values < 0 indicate an error, with a global table of error conditions and error messages
.PP
The PMAPI routine
.BR pmErrStr (3)
translates error conditions into error messages.
By convention, the small negative
values are assumed to be negated versions of the Unix error codes as defined
in
.B <errno.h>
and the strings returned are as per
.BR strerror (3).
The larger, negative error codes are PMAPI error conditions.
.PP
One error, common to all PMAPI routines that interact with
.BR pmcd (1)
on some host is
.BR PM_ERR_IPC ,
which indicates the communication link to
.BR pmcd (1)
has been lost.
.SH "MULTI-THREADED APPLICATIONS"
The original design for PCP was based around single-threaded applications, or
more strictly applications in which only one thread was ever expected to
call the PCP libraries.
This restriction has been relaxed for
.B libpcp
to allow the most common PMAPI routines to be safely called from any
thread in a multi-threaded application.
.PP
However the following groups of functions and services in
.B libpcp
are still restricted to being called from a single-thread, and this is enforced
by returning
.B PM_ERR_THREAD
when an attempt to call the routines in each group from more than one
thread is detected.
.TP 4n
1.
Any use of a
.B PM_CONTEXT_LOCAL
context, as the DSO PMDAs that are called directly from
.B libpcp
may not be thread-safe.
.SH "PCP ENVIRONMENT"
Most environment variables are described in
.BR PCPIntro (1).
In addition,
environment variables with the prefix
.B PCP_
are used to parameterize the file and directory names
used by PCP.
On each installation, the file
.I /etc/pcp.conf
contains the local values for these variables.
The
.B $PCP_CONF
variable may be used to specify an alternative
configuration file,
as described in
.BR pcp.conf (5).
Values for these variables may be obtained programmatically
using the
.BR pmGetConfig (3)
function.
.SH SEE ALSO
.BR PCPIntro (1),
.BR PCPIntro (3),
.BR PMDA (3),
.BR PMWEBAPI (3),
.BR pmGetConfig (3),
.BR pcp.conf (5),
.BR pcp.env (5)
and
.BR PMNS (5).
